1. Field of the Invention
The present invention relates to a variable optical filter unit for varying a wavelength characteristic of incident signal light and a variable gain equalizing system for compensating for wavelength characteristic variation in input signal light.
2. Description of the Related Art
Recently, research and development has been made on Wavelength Division Multiplexing (WDM) long-distance optical transmission in the field of optical communications. In the WDM long-distance optical transmission system, a plurality of optical amplifiers (e.g. EDFAs (Erbium-Doped Optical Fiber Amplifiers)) are inserted on the optical transmission line.
However, because the optical amplifier in a use wavelength band is different in gain depending upon a wavelength, the signal light passed the optical amplifier is different in light amount due to the wavelength. In the WDM long-distance optical transmission system, because the signal light passes a plurality of optical amplifiers, there is cumulative increase of difference in signal light amount between the wavelengths. This causes great difference in signal light amount between wavelengths on channels. This results in difference in the ratio of noise magnitude to light amount of the signal (S/N ratio) between channels (wavelengths) and hence makes impractical.
The countermeasure to the problem resulting from the optical-amplifier gain characteristic includes, for example, the means to incorporate a gain equalizer individually in each optical amplifier or insert a plurality of gain equalizers on the optical transmission line so that the difference in signal light amount between wavelength caused by the optical amplifiers can be compensated for by the gain equalizers thereby equalizing the signal light amount on every wavelength.
Conventionally, the gain equalizer has been configured with using an optical filter (e.g. dielectric multi-film filter, etalon filter, Mach-Zehnder filter) having a fixed light attenuation amount wavelength characteristic as a relationship between a light attenuation amount and a wavelength, on the assumption that the optical amplifier is steady in gain characteristic.
However, actually the optical-amplifier gain characteristic is not steady but varies due to the variation in input-signal light amount resulting from aging deterioration of the optical fiber constituting the optical transmission line or cut-working of the optical fiber upon extension of optical transmission line, or the variation in excitation state of the optical amplifier medium resulting from external disturbance such as temperature change in the environment of installing the optical amplifiers. Consequently, there are cases that the difference in input signal light amount between wavelengths cannot be satisfactorily compensated for by the gain equalizer utilizing an optical filter fixed in light attenuation amount wavelength characteristic.
Considering the above, there have been recent proposals on the variable gain equalizers capable of varying the light attenuation amount wavelength characteristic. Various variable gain equalizers have been proposed, e.g. variable gain equalizers under thermo-optical control on the principle of a waveguide-type diffraction grating and Mach-Zehnder interferometer, variable gain equalizers with mechanical means on the principle of a split-beam Fourier filter, variable gain equalizers for gain-inclination correction comprising a variable optical filter unit on the principle of a birefringence filter having a sinusoidal-like filter characteristic, variable gain equalizers comprising a plurality of such variable optical filter units arranged in series and so on.
Of the plurality of proposed variable gain equalizers, the present inventor has an attention to the variable gain equalizer using the variable optical filter unit. The reason lies in, first, that the variable gain equalizer has a high reliability in variably controlling light attenuation amount wavelength characteristic because of a structure to variably control light attenuation amount wavelength characteristic under electric control. Second, the variable optical filter unit is easy to manufacture and low in price because of the capability of manufacturing by a bulk structure that optical elements are arranged in a free space to provide action to transmission light. Third, where the plurality of variable optical filter units are arranged in series to structure a variable gain equalizer, light attenuation amount wavelength characteristic can be varied in various ways by individually controlling the variable optical filter units. Thus, light attenuation amount wavelength characteristic is high in freedom in variable setting.
FIG. 5 typically shows a system example of a variable gain equalizer using the plurality of variable optical filter units. The variable gain equalizing system 1, shown in FIG. 5, is structured with a plurality of variable optical filter units 2 (21, 22, . . . 2n (n is an integer equal to or greater than 2)), monitor means 3 and pattern form control means 4. The variable optical filter units 2 each possess a sinusoidal-like light attenuation amount wavelength characteristic. The variable gain equalizing system 1 is configured for producing a compensating light attenuation amount wavelength characteristic as shown at the curve Axe2x80x2 in FIG. 6B for compensating for am input-signal-light (incident signal light) wavelength characteristic, e.g. as shown at the curve A in FIG. 6A, by adding together the sinusoidal-like light attenuation amount wavelength characteristics of the variable optical filter units 2, to output signal light nearly equal in light amount at every wavelength as shown in FIG. 6C. Note that the sinusoidal-like includes not only a sinusoidal-like form but also the waveforms approximate in form to the sinusoidal-like form throughout this specification.
In the variable gain equalizing system 1, for example the monitor means 3 monitors a wavelength characteristic of an input signal. Depending on the result of monitor, the pattern-form control means 4 individually controls the variable optical filter units 2 to control at least one of amplitude and phase of the sinusoidal-like light attenuation amount wavelength characteristic of each variable optical filter unit 2, thereby variably controlling the light attenuation amount wavelength characteristic for compensating the variable gain equalizing system 1. specifically, where for example input signal light has a waveform characteristic as shown at curve A in FIG. 6A, produced is a light attenuation waveform characteristic for compensation as shown at the curve Axe2x80x2 in FIG. 6B. Meanwhile, where the input signal light is changed to a waveform characteristic as shown at the curve B in FIG. 6A, the pattern-form control means 4 individually controls the variable optical filter units 2, to variably control the amplitude or phase of the sinusoidal-like light attenuation waveform characteristic of each of the variable optical filter units 2. By adding together the sinusoidal-like light attenuation amount wavelength characteristics of the variable optical filter units 2, a light attenuation amount wavelength characteristic for compensation is produced as shown at the curve Bxe2x80x2 in FIG. 6B.
The variable gain equalizing system 1 shown in FIG. 5, capable of changing the light attenuation amount wavelength characteristic for compensation in various ways as in the foregoing, can output signal light nearly constant in light amount regardless of wavelength as shown in FIG. 6C even where the wavelength characteristic of input signal light varies.
FIG. 7 typically shows a configuration example of a variable optical filter unit 2 for constituting the variable gain equalizing system 1. The variable optical filter unit 2 shown in FIG. 7 is configured to have a polarizer 6, a Faraday rotator 7, a linear retarder 8, a Faraday rotator 9 and a analyzer 10 arranged in the order on a light propagation path. Further, provided are magnetic-field applying devices 11, 12 to apply magnetic field to the Faraday rotators, 7, 9 and temperature control device 13 to control the temperature of the linear retarder 8.
The Faraday rotator 7, 9, structured of magneto-optical crystal, e.g. YIG (Yttriun-iron-garnet), utilizes the Faraday effect to rotate a polarization state of input light depending on a magnitude of magnetization in a light propagation direction due to a magnetic field applied by the magnetic-field applying devices 11, 12. The magnetic-field applying means 11, 12 variably control the magnitude of a magnetic field applied to the Faraday rotators 7, 9 thereby variably controlling the magnitude of magnetization in a light propagation direction in the Faraday rotator 7, 9 and hence variably controlling the rotation angle (Faraday rotation angle) xcex8 in the polarization state of input light by the Faraday rotators 7, 9.
In the meanwhile, there exist microscopic gatherings of magnetization called magnetic domains in a magneto-optical crystal (multi-domain structure). If a magnetic field is externally applied, the magnetic domain grows gradually into a greater magnetic domain. Finally, the magnetic domains are integrated into a state of saturated magnetization. In the state of multi-domain structure, optical transmission loss occurs due to diffraction loss caused by a multiplicity of domain boundaries. In order to reduce optical transmission loss, the magneto-optical crystal is desirably used in a saturated domain state.
Consequently, the magnetic-field applying devices 11, 12 preferably uses magnetic-field applying devices capable of variably controlling the magnitude of magnetic domains in the Faraday rotators 7, 9 in the light propagation direction while maintaining the saturated domain state of the Faraday rotators (magneto-optics crystal) 7, 9. Such a magnetic-field applying device includes various structures. Herein, any of the structures of magnetic-field applying device may be employed, and hence explanation thereof is omitted.
Note that the magnetic-field applying devices 11, 12 are controlled such that the Faraday rotators 7, 9 are equal in Faraday rotation angle xcex8, because of the reason hereinafter referred.
The linear retarder 8, formed of a birefringence crystal, e.g. quartz or rutile crystal, is a device to separate, with a phase difference, transmission light into a component polarizing in an optical-axis direction of the crystal and a component polarizing in a direction orthogonal to the optical-axis direction of the crystal. The temperature control device 13, for variably controlling the temperature of the linear retarder 8 itself, is structured, for example, by a Pertier device.
In the meanwhile, simulations have been made, using computation with Jones matrix, on how the amount of the signal light of after passing the optical variable filter unit 2 varies in the case of varying the Faraday rotation angle xcex8 of the Faraday rotators 7, 9 and in the case of varying the temperature of the linear retarder 8. Due to this, it has been confirmed that the light attenuation amount wavelength characteristic of the variable optical filter unit 2 varies, as shown in FIG. 8, in the case the Faraday rotation angle xcex8 is varied, and as shown in FIG. 9, in the case the temperature of the linear retarder 8 is varied.
The solid line A shown in FIG. 8 is for a Faraday rotation angle xcex8 of 45xc2x0, the broken line B for a Faraday rotation angle xcex8 of 55xc2x0, the broken line C for a Faraday rotation angle xcex8 of 60xc2x0, the dotted line D for a Faraday rotation angle xcex8 of 75xc2x0 and the solid line E for a Faraday rotation angle xcex8 of 90xc2x0.
As can be seen from the graph on the light attenuation amount wavelength characteristic shown in FIG. 8, the variable optical filter unit 2 has a sinusoidal-like light attenuation amount wavelength characteristic wherein it is seen that the sinusoidal-like waveform varies only in amplitude with variation in the Faraday rotation angle xcex8 without change in phase and period.
The solid line A shown in FIG. 9 is on a waveform example of a light attenuation waveform characteristic in the variable optical filter unit 2 in a case the linear retarder 8 is at a temperature of 10xc2x0 C. The dotted line B is on a case the linear retarder 8 is at a temperature of 20xc2x0 C., and the dotted line C is on a case the linear retarder 8 is at a temperature of 30xc2x0 C. As can be seen from the graph on the light attenuation waveform characteristic, it can be conformed that the sinusoidal-like waveform on the light attanuation waveform characteristic of the variable optical filter unit 2 varies in phase with variation in the temperature of the linear retarder 8 with the period unchanged.
As shown in FIGS. 8 and 9, the sinusoidal-like characteristic exhibited in the light attenuation waveform characteristic of the variable optical filter unit 2 is due to variation depending on a wavelength because the phase difference xcex94 in the separation light by the linear retarder 8 is of wavelength dependency. Also, when the temperature of the linear retarder 8 changes, the birefringence of the linear retarder 8 changes to vary the phase difference xcex94 of the separation light by the linear retarder 8 and accordingly the phase in the light attenuation amount wavelength characteristic of the variable optical filter unit 2 varies. On the contrary, the reason of no change of the period in the light attenuation amount wavelength characteristic of the variable optical filter unit 2 by temperature change in the linear retarder 8 is because the variation amount in the separation-light phase difference xcex94 due to the temperature change is not dependent upon a wavelength and nearly constant at every wavelength.
Namely, in the variable optical filter unit 2 shown in FIG. 7, the amplitude of the sinusoidal-like light attenuation amount wavelength characteristic of the variable optical filter unit 2 can be controlled by variably controlling the Faraday rotation angle xcex8 of the Faraday rotators 7, 9. Also, by variably controlling the temperature of the linear retarder 8, the phase of the sinusoidal-like light attenuation amount wavelength characteristic of the variable optical filter unit 2 can be controlled. Namely, the temperature control means 13 functions as a phase difference varying device for controlling the separation-light phase difference xcex94 due to the linear retarder 8 to thereby vary the phase of the sinusoidal-like light attenuation amount wavelength characteristic of the variable optical filter unit 2.
Furthermore, the period of the sinusoidal-like light attenuation amount wavelength characteristic of the variable optical filter unit 2 is determined depending upon a crystal thickness in the light propagation direction of the linear retarder 8. Previously determined is a crystal thickness d in the light propagation direction of the linear retarder 8 for a predetermined period of the light attenuation amount wavelength characteristic. The linear retarder having a determined thickness d will be provided in the variable optical filter unit 2. Incidentally, although the crystal thickness in the light propagation direction of the linear retarder 8 is varied by variably controlling the temperature of the temperature control means 13, such variation is negligibly small and accordingly, if the temperature of the linear retarder 8 is varied, the period of the light attenuation amount wavelength characteristic will nearly not changed.
The polarizer 6 and the analyzer 10 are, respectively, configured by linear polarizers each comprising, e.g. a polarizing splitting wedge using a birefringence crystal not to change the amount of the light of after transmission by an incident light polarizing state.
The polarizer 6, the linear retarder 8 and the analyzer 10 are preferably arranged in the following relationship so that all the peaks on the sinusoidal-like waveform of the light attenuation amount wavelength characteristic of the variable optical filter unit 2 are rendered zero in transmission loss on principle as shown in FIG. 8 or FIG. 9. For example, when the polarizer 6 and the analyzer 10 are in an othogonal-Nicol relationship, the linear retarder 8 is provided such that the birefringence crystal constituting the linear retarder 8 in an optical axis direction inclines 45xc2x0 relative to a direction of transmission through the polarizer 6 and analyzer 10. Meanwhile, when the polarizer 6 and the analyzer 10 are in a parallel-Nicol relationship, the linear retarder 8 is provided such that the crystal of the linear retarder 8 in the optical axis direction is parallel with a direction of transmission through the polarizer 6 and analyzer 10.
Also, the reason for controlling the magnetic-field applying device 11, 12 such that the Faraday rotators 7, 9 are equal in Faraday rotation angle xcex8 as in the foregoing is in order to render zero in transmission loss on principle all the peak values on the sinusoidal-like waveform of the light attenuation amount wavelength characteristic of the variable optical filter unit 2, similarly to the above. In this manner, the reason for rendering all the peak values on the sinusoidal-like waveform of the light attenuation waveform characteristic zero in transmission loss is in order to reduce the amount of transmission loss in the variable gain equalizing system 1 as low as possible when the variable gain equalizing system 1 is architected by connecting a plurality of variable optical filter units 2 in series as shown in FIG. 5.
Note that the equal Faraday rotation angle xcex8 in the Faraday rotators 7, 9 is in order for simplifying the configuration of control, besides the above reason.
Meanwhile, in the above example, the temperature control means 13 of the linear retarder 8 variably control the phase in the sinusoidal-like light attenuation waveform characteristic of the variable optical filter unit 2. However, it is possible to interpose a variable retarder using a Faraday rotator between the Faraday rotator 9 and the analyzer 10 without providing a temperature control means 13 as disclosed in JP-A-6-130339 so that the variable phaser can variably control the phase in the light attenuation amount wavelength characteristic of the variable optical filter unit 2.
In the meanwhile, there is a proposal on a variable optical filter unit 2 as shown in FIG. 10 in view of reducing the size and cost of the variable optical filter unit 2. The variable optical filter unit 2 shown in FIG. 10 has a polarizer 6, a Faraday rotator 7, a linear retarder 8, and a total reflecting mirror 18 as a total reflecting element, arranged in the order of light propagation direction. Also, provided are a magnetic-field applying device 11 for applying a magnetic field to the Faraday rotator 7 and a temperature control device 13 for variably controlling the temperature of the linear retarder 8. Note that, herein, the same structural parts as those of the variable optical filter unit 2 shown in FIG. 7 are denoted with the same reference numerals to omittedly explain the duplicated common parts.
In the variable optical filter unit 2 shown in FIG. 10, the light passed the polarizer 6 reaches the total reflecting mirror 18 through the Faraday rotator 7 and the linear retarder 8 in the order where it is totally reflected by the total reflecting mirror 18. The returning light is outputted from the polarizer 6 through the linear retarder 8 and the Faraday rotator 7 in the reverse order to the above. The variable optical filter unit 2 shown in FIG. 10 can act upon light to produce a sinusoidal-like light attenuation amount wavelength characteristic, similarly to the variable optical filter unit 2 shown in FIG. 7.
Incidentally, the polarizer 6 serves also as the analyzer 10 structuring the variable optical filter unit 2 shown in FIG. 7. In the case of FIG. 10, the polarizer 6 and the analyzer 10 are in a state equivalent to a parallel-Nicol relationship. Accordingly, the linear retarder 8 is preferably arranged in its crystal optical axis direction parallel with a direction of transmission through the polarizer 6.
In the meanwhile, it can be considered that a collimator 19 as shown in FIG. 11 be arranged at a light input/output section of the variable optical filter unit 2 shown in FIG. 10. The collimator 19 is integrated with a two-cored ferrule 20 and a lens 22. The two-cored ferrule 20 has optical-fiber cores/clads 21a, 21b arranged side by side through a spacing (e.g. 250 xcexcm). One of the cores/clads 21a, 21b serves as an input optical fiber while the other as an output optical fiber.
In the case that such a collimator 19 is arranged at the light input/output section of the variable optical filter unit 2 of FIG. 10, the light outputted at a side serving as an input optical fiber of the cores/clads 21a, 21b is incident on the polarizer 6 through the lens 22. Then, as in the foregoing, the returning light totally reflected upon the total reflecting mirror 18 passes the lens 22 to enter and propagates to the other core/clad on the other side serving as the output optical fiber.
In the collimator 19 shown in FIG. 11, the cores/clads 21a, 21b are arranged symmetric about, as a center, an optical axis of the lens 22. In this case, in order that the light entered one of the cores/clads 21a, 21b is reflected upon the total reflecting mirror 18 and the returning light thereof enters the other core/clad, there is a need to make a spacing fa between a tip of the core/clad 21a, 21b and a principal plane of the lens 22 equal to a spacing fb between the principal plane of the lens 22 and the total reflecting mirror 18.
In the meanwhile, it is preferred to use a general-purpose collimator in consideration of cost reduction. However, in a general-purpose collimator, the spacing fa between the tip of the core/clad 21a, 21b and the principal plane of the lens 22 is approximately 1-4 mm. In order to employ a general-purpose collimator, there is a need to make the spacing fb between the lens 22 principal plane and the reflecting mirror 18 approximately 1-4 mm. However, there is extreme difficulty in arranging, in such a narrow gap, a polarizer 6, a Faraday rotator 7 and a linear retarder 8 and further a magnetic-field applying device 11 and a temperature control device 13.
This makes it impossible to use a general-purpose collimator. The use of an expensive collimator incurs cost increase for a variable optical filter unit 2 and a variable gain equalizing system using the same.
Meanwhile, for example, in the case of fabricating a collimator 19 by setting the spacing fb between the principal plane of the lens 22 of the collimator 19 and the total reflecting mirror 18 in order to facilitate the arrangement of the optical elements such as the Faraday rotator 7 as well as the spacing fa between the lens 22 and the tip of the core/clad 21a, 21b to an equal to the spacing fb, the collimator 19 is greater in size as compared to the general-purpose product, resulting in size-increase in a variable optical filter unit 2 and variable gain equalizing system 1.
Furthermore, it can be considered that, in order to avoid size increase of the collimator 19, optical elements such as a polarizer 6 be inserted between the lens 22 and the tip of the core/clad 21a, 21b. In this case, there is difficulty in adjusting the positions of the lens 22, the optical elements such as the polarizer 6 and tip of the core/clad 21a, 21b such that minimized is the optical coupling loss of the returning light totally reflected by the total reflecting mirror 18 with the core/clad 21 a or 21b, lowering production efficiency. In this case, the collimator 19 is expensive despite avoiding size increase of the collimator 19, resulting in cost increase of a variable optical filter unit 2 and variable gain equalizing system 1 similarly to the foregoing.
The present invention has been made in order to solve the above problem, and it is an object to provide a variable optical filter unit and variable gain equalizing system low in price and small in size by using a general-purpose collimator.
In order to achieve the object, a variable optical filter unit having a sinusoidal-like light attenuation amount wavelength characteristic having a integrated collimator with an input optical fiber and an output optical fiber that are arranged side by side and a lens arranged with a spacing to a tip of the input and output optical fibers commonly for input and output, the variable optical filter unit comprising: arranged on a light exit side of the collimator, in an order, a polarizer; and a total reflecting element for totally reflecting a signal light to return a propagation direction of the light; provided between the polarizer and the total reflecting element a Faraday rotator for rotating a polarizing plane of an incident light according to an applied magnetic field; a birefringence crystal for providing, depending on a wavelength, a phase difference between a component propagating with polarization in a crystal optical axis direction and a component propagating with polarization in a direction orthogonal thereto; and a phase difference changing device for changing, without depending on the wavelength, the phase difference between the component propagating with polarization in the crystal optical axis direction and the component propagating with polarization in the direction orthogonal thereto; a propagation light path changing element being provided to input and propagate a returning light totally reflected by the total reflecting element onto the output optical fiber of the collimator. This structure is means for solving the foregoing problem.
Also, a variable optical filter unit having a sinusoidal-like light attenuation waveform characteristic having a collimator integrated with a tip of an optical fiber and a lens arranged on a side of the tip of the optical fiber through a spacing, the variable optical fiber filter unit comprising: arranged on a light exit side of the collimator, in an order, a polarizer; and a total reflecting element for totally reflecting a signal light to return a propagation direction of the light; provided between the polarizer and the total reflecting element a Faraday rotator for rotating a polarizing plane of an incident light according to an applied magnetic field; a birefringence crystal for providing, depending on a wavelength, a phase difference between a component propagating with polarization in a crystal optical axis direction and a component propagating with polarization in a direction orthogonal thereto; and a phase difference changing device for changing, without depending on the wavelength, the phase difference between the component propagating with polarization in the crystal optical axis direction and the component propagating with polarization in the direction orthogonal thereto; a propagation path of the returning light totally reflected by the total reflecting element and directed toward the collimator being coincident with a propagation path of the outgoing light directed from the collimator toward the total reflecting element, an optical circulator being inserted on an optical fiber connected to the collimator.